I use OpenRC rather than systemd, but I doubt the problem is due to systemd per se. I suspect it's due to the way SL has been configured, as I assume the 'kdesu dolphin' command works in other distributions that use systemd (such as Fedora or Arch).

The 'xhost +' command allows all users to make connections to the currently-running X Server (as your error message indicates that the root user or kdesu does not have permission to connect to the running X Server). The command 'dbus-launch' is because a D-Bus daemon is apparently not running as root user (which could be a configuration issue in SL). Maybe if a SL developer with systemd experience reads this he would be able to shed some light on it. You might want to file a bug report.